a. Field of the Invention
This invention relates to a process for electrolytically coloring aluminum or any of its various alloys a gold color.
B. Prior Art
A variety of processes have been developed for coloring aluminum and aluminum alloys. Typical examples of such known processes include one wherein an oxide film formed anodically on a basis metal is dipped in a solution of an organic dye or inorganic compound; another wherein aluminum alloy is colored by its own composition or by bath composition; and still another wherein an anodized basis metal is electrolyzed by use of alternating current in a bath containing a metallic salt, with the consequent deposition of the metal or metal oxide on the oxide film on the basis metal.
The first described dip coloring process has drawbacks such as the low weather resistance of the color produced and the irregularity of coloring due to fluctuations in bath temperature or in film thickness. The second described process, in which workpieces become colored simultaneously as coatings are formed thereon by electrolysis, has difficulties in connection with production of homogeneous alloy. Additional problems include the irregularity of coloring due to uneven coatings produced, the need for the use of high voltages, and high expenses required.
Most widely practiced has been the third process mentioned above, wherein anodized workpieces have their oxide films colored by electrolysis in a metallic salt bath with the use of alternating current. For coloring aluminum or aluminum alloy in gold, the principles of this third process have been utilized in several proposed methods. Since the metallic salts used by these prior art methods are either expensive or noxious, such methods are disadvantageous from the standpoints of production, bath control, and pollution. Another disadvantage is that the gold colored coatings produced by these methods are often poor in weather resistance.
It has also been suggested to use a bath of tin salt or of its aqueous solution in electrolytically coloring anodized aluminum or aluminum alloy by use of alternating current. According to this known process, however, the basis metal can only be colored light beige, olive, bronze, deep red, black and so forth, but not gold, no matter how the concentration of the metallic salt and other conditions of electrolysis are controlled.